A number of medical devices are available which provide assistance in the performance of various physiological functions for an individual experiencing difficulties. Some of these devices are implanted within the patient's body while others are external and are typically utilized under temporary or emergency situations. Whether residing inside or outside of the patient's body, many devices include an electrode or cable connected to the patient for the performance of a physiological function or for the receipt of information from the patient. Cardiac pacemakers, defibrillators, the Jarvis heart and insulin pumps are a few examples of devices which may be implanted within the patient's body. External devices also include cardiac pacemakers and defibrillators and ECG monitors and other diagnostic equipment.
External pacemakers, defibrillators and the like are usually portable devices which are battery-powered and capable of being recharged. The device's portability is important because it is typically used in emergency situations or as a temporary resolution to a patient's problems before a more permanent solution can be rendered. For example, defibrillators may be carried in an ambulance or used in hospital emergency rooms to revive a patient or to restore a normal heart rhythm. An external pacemaker may be used to provide assistance to a patient who has not yet had an internal pacemaker implanted or to provide pacing while the patient is in surgery to implant or replace an implantable pacemaker.
To provide signals to the medical electrodes, or to receive signals from the patient as is necessary for monitoring, diagnostic or other functions of the device, cables, electrodes or wires extend from the device to the patient. For example, the Medtronic Model 5345 Temporary Pulse Generator, available from Medtronic, Inc. of Minneapolis, Minn., is a temporary pacemaker such as may be used by a patient prior to implanting a permanent pacemaker. A patient cable is connected to the device and unipolar or bipolar pacing leads contacting the patient are connected to the patient cable. The Medtronic model 5311 pacing system analyzer is used to test sensing and pacing thresholds of heart electrodes. The Code Master Defibrillators, available from Hewlett Packard of Andover, Mass., provide the capabilities of defibrillation and ECG monitoring. Both integral defibrillator paddles and optional gel electrodes may be used with the device to accomplish these functions. Similarly the Lifepak.RTM. 6 Cardiac Care System, available from Physio-Control.RTM. Corporation of Redmond, Wash., uses paddles for defibrillation and patient cables for monitoring ECG. A combination pacemaker and defibrillator such as the Zoll PD.TM. 1200 Pacemaker/Defibrillator, available from ZMI Corporation of Woburn, Mass., uses paddles or gel electrodes for defibrillation and gel electrodes for pacing and for monitoring ECG signals. In all instances, the integrity of the defibrillator, patient cable, or electrode is of utmost importance for the proper operation of the external device.
Presently, most portable pacemakers and defibrillators are periodically checked by hospital staff to determine only whether the most basic functionality exists. For example, a nurse may check such a unit to determine whether it is plugged in, whether the proper display appears when the unit is switched on, and whether an appropriate quantity of ancillary supplies, such as pacing pads, are readily available. However, other aspects of the system do not lend themselves to being readily checked. For example, many connectors between the cables, leads, electrodes or pads of such systems may become loose or bent, resulting in a poor or open circuit. For such systems, there may be no apparent indication that the equipment is faulty, causing the defective equipment to be used on patients in critical condition. The only way to detect such an equipment failure is for an astute medical technician to notice that all of the patients on which the defective equipment had been recently used had died, and to have the equipment thoroughly inspected by an advanced medical technician.
Some external devices, such as the Code Master Defibrillators and the Zoll PD.TM. 1200 Pacemaker/Defibrillator, provide information to the operator or technician regarding the "integrity" of the electrode. Specifically, these systems provide a warning when a complete circuit is not provided. A complete circuit is one in which there is current flow between the electrodes and there are no severe discontinuities in the electrodes. Thus, the electrodes must be place against a patient's body to complete an electrical circuit in order to determine whether a break exists in the cables, electrodes or the connectors of either. Because the cables or electrodes must contact the patient before a test may be performed does not allow a fault in the system to be identified until the system must be used. Furthermore, because time is of the essence when the device is utilized on a patient, it is desirable to provide an integrity testing system for the electrodes or cables connected to such external devices which does not require that the patient be connected to the electrodes or cables so that the cables or electrodes may be tested on a routine basis. In addition, it is desirable to provide a system that not only indicates whether there is an open circuit in a medical electrical system, but location of the open circuit, e.g. whether the break occurs where the cables plug into the pulse generating unit, the connector between the cables and the pacing pads, or in the middle of the wires of any of the cables.
Many of the electrodes used with these external devices are disposable. A sterilized package containing the electrodes is opened just prior to use. Thus, it is also desirable to provide an electrode integrity testing system which is time efficient so as to avoid delaying the provision of the assistance necessary. Furthermore, because physicians will often be utilizing an integrity testing system under stressful circumstances, it must be easy to use and the results of the test must lead to quick interpretation of the integrity of the electrodes or cables.
It is also possible for a problem to occur with the integrity of the cables or electrodes during their use. For example, leads may become disconnected from the patient or from the device or the cable to which the lead is connected. Therefore, it is desirable to provide an integrity testing system which can be operated during the device's operation and which does not interfere with the device's other functions.
Time domain reflectometers, such as the 1502C Metallic Time Domain Reflectometer manufactured by Tektronix, Inc. of Beaverton, Oreg., are used to test the integrity of cable such as co-axial cables. For such integrity testing, time domain reflectometers send electrical pulses down the cable and detect any reflections may by any discontinuities in the cable. Specifically, time domain reflectometers send out successive pulses and measure the respective reflected pulses at times corresponding to points along the cable. Measurements are provided in terms of voltage versus time which can then be converted to resistance over the length of the cable. Time domain reflectometers can locate shorts, opens, defects in the shield of the cable, foreign substances in the cable, kinks, and more. Generally, only one parameter is required for the proper operation of the time domain reflectometer in determining the integrity of a cable. That parameter is the velocity of propagation or the speed of the signal down the cable which varies for different cable dielectric materials. Time domain reflectometers may operate on either a closed or an open circuit. For an open circuit the signal continues to be reflected from the wire and returns to the instrument. In general, variations in the resistance measured by the time domain reflectometer indicates a fault such as a bad connection, the stripping of insulation, pressure on the cable, or a break in the cable.
Time domain reflectometry has been used for a variety of applications. In U.S. Pat. No. 4,466,288, time domain reflectometry is used to evaluate vibrations. The level of fluid in a vessel may be determined by time domain reflectometry as disclosed in U.S. Pat. No. 3,922,914. Also, the constituents of a multi-phased fluid system have been evaluated as disclosed in U.S. Pat. No. 4,786,857.
In addition, time domain reflectometry has been used for optical systems as well. For example, optical time domain reflectometers, such as that disclosed in U.S. Pat. No. 4,960,989, may be used to determine the tip location of a consumable electrode within an electric furnace as disclosed in U.S. Pat. No. 4,843,234. Similarly, optical time domain reflectometry is used in U.S. Pat. No. 5,033,826 to determine which surface of a photographic lens is impairing transmissivity.
U.S. patent application Ser. No. 07/866,850, filed Apr. 10, 1992, discloses an integrity testing system for implantable electrodes such as may be utilized with implanted pacemakers or defibrillators. The electrical device disclosed in U.S. patent application Ser. No. 07/866,850 has a receptacle for an electrode and includes a time domain reflectometer comprising an output signal mechanism operatively connected to the electrode receptacle. The method of analyzing the integrity of an implanted electrode comprises the steps of generating a time domain reflectometer (TDR) reading (output signal) from the electrode, and transmitting the signal to an output device. The output signal may be analyzed to determine whether it varies by a predetermined threshold, or it may be compared to a previously generated signal from the electrode so that differences in the electrical characteristics of the electrode may be identified. The invention allows a cardiologist or technician to ensure that the electrode is properly implanted and to non-invasively determine the integrity of the electrode over a patient's life.
It is desirable to provide a method and device using time domain reflectometry to determine the integrity of electrodes or cables connected to an external device to thereby alert the cardiologist or the technician of a potential or existing problem associated with the electrode or cable. As indicated above, time domain reflectometry may be used with both unipolar or bipolar electrodes. The velocity of propagation of any electrode is necessary for time domain reflectometry measurements. Such a parameter could be stored in the device.
It is also desirable to provide a method of analyzing the integrity of the electrode or cable connected to the external device. Such analysis could be completed in within the device. Of course, to be used in conjunction with the portable devices, the integrity testing system must be able to be integrated within the device and must not consume a great deal of power.